Nuclear reactor cores used for power generation typically have a large number of fuel assemblies arranged in suitable configuration to heat water, thereby producing steam for turning electrical generators. Each of such fuel assemblies typically contains from 49 to as many as 300 fuel rods which contain nuclear fuel. A larger nuclear reactor may contain as many as 40,000 to 50,000 fuel rods.
Each fuel rod is operatively independent of other fuel rods within a fuel assembly and is basically a metal tube approximately 0.5 inches in diameter that extends typically from 8 to 15 feet in length. Suitable fissionable material such as uranium oxide in the form of cylindrical fuel pellets is stacked within the fuel rod. The upper end of the tube is void of fuel pellets and forms a plenum, when sealed, for gas. A small clearance space is provided around the fuel pellets to accommodate expansion or swelling of the fuel. Since the nuclear fuel within the fuel rods becomes extremely radioactive during operation and is contained only by the metal tubes or sheaths of the rods, inspection methods and apparatus for verifying the seal integrity of the rods is of primary importance. During prolonged operation, the fuel rods may develop leaks as a result of cracks, pinholes or other defects, resulting in coolant seeping into the fuel rod and/or the escape of radioactive products from the fuel rod. A leakage of fission product gases and other radioactive materials from a fuel rod during reactor operation can enter the reactor coolant system. While the coolant purification system of a reactor facility is capable of handling a certain amount of radioactive fission products, it is desirable to keep the radiation level as low as possible.
In early fuel assembly designs, the individual fuel rods were part of a unitized assembly wherein the fuel rods were welded together by fuel assembly guides in a manner making it very difficult to replace a failed fuel rod. More recent designs have constructed fuel assemblies in a manner such that a fuel rod that fails before its fissionable material is spent can be removed or replaced by a new rod. Such fuel assembly design, generally referred to as a reconstitutable fuel assembly, facilitates removal and replacement of fuel rods and saves considerable expense heretofore incurred as a result of premature discharge of an entire fuel assembly because of the failure of a single one of its fuel rods.
A key step in the fuel assembly reconstitution process is the identification of the specific fuel rod or rods of the fuel assembly which have failed due to cracks, pinholes or other penetrations of the tube or cladding of the fuel rod. The first step in such process is to identify a fuel assembly containing one or more leaking fuel rods. This operation is typically performed during reactor refueling. During refueling operations, the reactor is shut down, and the reactor fuel assemblies of the reactor are usually subjected to a so-called "sipping test". The fuel assembly under test is placed in a water filled storage tank. The fuel rods of the assembly and the water in the storage tank heat up by residual decay. If a fuel assembly contains a defective rod, the fission products from that rod escape through the defect into the water. Through sampling of the water, it can be determined whether the fuel assembly contains any defective rods. Such sipping test is a totalizing method that only determines whether the entire fuel assembly contains any defective rods, and does not identify the position or location of the defect or the defective rod per se. While a number of methods and devices have been utilized in the past for detecting and locating single fuel rod failures, such techniques of the prior art have generally not provided the desired simplicity, reliability, speed, safety and cost effectiveness for such operation.
A desired rod failure detection method should provide identification of the defective rod, without requiring any significant design changes to the fuel, to the rod itself or to the fuel assembly. In short, it is most desirable that the rod failure detection mechanism be capable of operation on existing fuel assembly and fuel rod designs. Further, the test apparatus and method used should not damage or increase the risk of further damage to the fuel rods or to the fuel assembly. The fuel rod detection method should provide a high probability of being able to readily distinguish between failed and good rods, should require little if any dismantling of the fuel assembly in order to perform the test, and should be capable of testing an entire fuel assembly in a reasonably short time, and preferably while such assembly is still immersed within the coolant fluid.
This inventon is distinguished from that body of prior art relating to fuel rod failure detection that requires the fuel rod itself to be constructed in a particular fashion or to include detectable structures or members that move or change their physical condition or parameters as a result of a leak in the fuel rod. Such art is exemplified, for example, by U.S. Pat. Nos. 3,666,625 and 4,217,173 to Nybo and Jabsen respectively. In the Nybo structure, the fuel rod is configured to include a magnetic device which moves within the fuel rod in the event of a rod failure. The Jabsen invention requires a specially designed fuel rod end cap having a hollow construction, in combination with a cellular end fitting for the fuel assembly, to enable radiation probes to be lowered into registry with the fuel rod end caps for measuring radioactivity emanating from the fuel rod, thereby providing an indication of the structural integrity of the rod. In contrast to the above-described art, the present invention can be used with fuel rods and fuel assemblies constructed therefrom of any standard configuration, as long as the elongate fuel rods are arranged in generally parallel, spaced-apart relationship within the fuel assembly.
Others have devised apparatus and methods for locating leaking fuel rods in a fuel assembly, without requiring specially constructed fuel rods or significantly modified fuel assemblies. Such methods and apparatus, however, have not generally satisfied the above desired criteria for such a system. For example, U.S. Pat. No. 4,193,843 to Womack et al. describes a leak detector apparatus that applies ultrasonic waves to the fuel rods and measures the fuel rod resonance response thereto, which is indicative of the water that leaks into the failed fuel rod. The Womack method presumes that a leaking fuel rod will have an accumulation of water coolant at its lower end, and only performs its test of the fuel rod at the lower portion of the rod. Such test may be accurate for gross rod failures, but does not necessarily detect smaller rod failures which have not yet resulted in accumulation of significant amounts of coolant within the rod. U.S. Pat. Nos. 4,016,749 and 4,039,376 to Wachter describe apparatus and methods for detecting the emission of bubbles expelled from defective rods. The fuel assembly is placed within a liquid bath in a manner such that the fuel rods are differentially pressurized greater than the liquid bath. Sensing means are provided to detect bubbles emitted from the rods and to correlate the position of the bubble on the surface of the liquid bath, to that of the defective rod submerged within the bath. The accuracy of such system to identify the leaking rod depends upon the bubble traveling vertically through the liquid bath to the bath surface, and upon the accuracy of the correlation apparatus. Since the lateral spacing between adjacent fuel rods in a fuel assembly can be as small as 0.05 inches, the sensing accuracy of the bubble detection apparatus must be extremely accurate, and any deviations of the bubble from a straight-line path to the coolant surface can further lead to unreliability of this system.
Another method known in the art for detecting one or more fuel rod failures within a fuel assembly is disclosed by U.S. Pat. No. 3,878,040 to Martucci. The Martucci apparatus delivers a stripping gas to the bottom of the fuel assembly which percolates up through the assembly entraining any gaseous fission products contained in the coolant. The stripping gas and entrained gases are then collected by a hood overlying the fuel assembly and measured for radioactivity content. While such apparatus can detect a leaking fuel assembly, it cannot determine which of the fuel rods in the assembly is defective.
Another leak detection apparatus requiring specially configured fuel rods and fuel assembly is disclosed in U.S. Pat. No. 4,192,373 to Wolowodiuk. The Wolowodiuk structure includes physical leak-detection connections permanently provided to each of the fuel rods, which can be monitored periodically for leaking conditions. Such apparatus requires a duplex wall fuel rod construction as well as a relatively complicated communication channel of tubes and bores to complete the leak detection system, and does not readily lend itself to a reconstitutable fuel assembly construction.
While such prior art structures and techniques for detecting fuel rod leaks have individually addressed various ones of the desired criteria for structures, no single device or method has simultaneously satisfied all the previously described criteria. In general, either the leak detection structures of the prior art have been unduly complex or required costly, expensive sensing or detection equipment, or have been unreliable or inaccurate, or too time consuming in performing the test function. The present invention effectively addresses and overcomes most of the above-mentioned deficiencies of prior art fuel rod leak detector and locating structures and methods. The detector apparatus and methods of this invention allow accurate location of defective fuel rods of a fuel assembly without requiring any design modification to existing fuel rods or fuel rod assemblies, and uses actual leak measurement techniques to locate a leaking rod as opposed to making predictions or assumptions as to the manner or location of fuel rod failure. Further principles of this invention can be used to test a fuel assembly without requiring removal of the fuel assembly from the coolant bath of the reactor facility in which it is normally submersed.